Upon pumping of a liquid, such as waste water comprising solid matter, by means of for instance a submersible pump, the solid matter will sooner or later have a negative influence on the capacity of the pump to transport the liquid. The solid matter stick fast in the hydraulic unit of the pump and attach slowly to the impeller of the pump as well as to the inner side of the pump house of the pump, and thereby the hydraulic efficiency of the pump is effected negatively and the pump will operate in a strained operation condition due to the increased rotary resistance, increased torque and deteriorated hydraulic properties. Today several known ways of more or less automatic cleaning of a pump when the pump, or more precisely the hydraulic unit of the pump, starts to clog. The strained operation condition is not directly detrimental for the pump, however an increased power consumption and worse pump performance is obtained, which is costly for the plant owner and which may result in negatively attendant effects such as flooded pump station when the accessible capacity of the pump is not enough to empty the pump station.
Known cleaning methods, or methods for controlling a pump arrangement, detects that cleaning is necessary and thereafter perform a predetermined standard cleaning sequence, that at least entail that the motor of the pump is decelerated by having the speed of the motor undergo a long predetermined ramping down driven by the control unit. It is known that one shall not stop the motor of the pump directly, especially due to requirements of avoiding so-called water hammer in the pipe system downstream the pump, but also due to the high torque and the big momentum that the impeller of the pump have during normal operation. If the motor is stopped directly water hammering will inevitable arise when the kinetic energy and the moment of inertia of the liquid in the pipes downstream the pump cause vibrations that risk to destroy the pipes and other construction elements, thereto it is an imminent risk that the impeller is loosen, the drive shaft of the pump is damaged, etc. Thus, a long and controlled ramping down of the speed of the motor always takes place.
A direct consequence of the absence of intelligence in the cleaning method is that the standard cleaning sequence used, and which is adequate during a strained operation condition as described above, drastically increase the load of the pump when a large and/or hard object enters the hydraulic unit of the pump and is wedged, i.e. when a detrimental operation condition for the pump arrangement has arisen. A detrimental operation condition mean an operation condition that immediately or in a short period of time will entail that the pump and/or the control unit will break. When the control unit, for instance in the form of a frequency converter (VFD), perform said ramping down when a large and/or hard object has become wedged and mechanically brake the impeller, the long and controlled ramping down of the motor force the impeller to rotate and the object is wedged harder/more severe. This entail in its turn that the impeller, drive shaft motor, etc. of the pump or the control unit will become overstrained and damaged.
In order to prevent the pump and/or the control unit to become damaged different safety systems/protective equipment, such as a safety disconnection breaker, fuses, etc., which are arranged to protect the equipment and trig before the equipment is damaged. Common for the above described detrimental operation conditions, i.e. if the safety system triggers and/or the pump arrangement break, service personal must perform an emergency turnout and take care of the fault/clogging. These turnouts are expensive and thereto an idle pump is costly for the plant owner.